Access and start systems of motor vehicles operate on the principle of an identification transmitter being verified by a vehicle-side transmitting and receiving device, which comprises an evaluation unit, in the motor vehicle. This verification is undertaken by the exchange of radio signals which typically lie in the low frequency range. The radio transmission is activated as soon as a vehicle user operates a vehicle door. In this case a code signal of the identification transmitter is compared in the evaluation electronics of the vehicle with known code signals. If there is a positive match between the signals the motor vehicle is opened or started. Because it is transmitted by radio, the signal can be measured, disrupted or used in other ways by third parties. This can lead to unauthorized use of the vehicle.
An anti-theft device for a motor vehicle is known from DE 198 36 957 C1 in which a transceiver unit emits two consecutive request signals and these outputs are measured in a code generator The two measured signals are sent back by the code generator to the transceiver unit and compared there. A security device controls access to the motor vehicle accordingly, based on the signal comparison.
An access authorization system for a motor vehicle is known from DE 198 12 294 C2. For this system two antennas are arranged on the long side of the vehicle, one on the driver's side in the wing mirror and another in the area of the rear side window, in order to be able to cover the largest possible area around the motor vehicle for transmitting and receiving.
A method for determining the position of an object and for controlling access to a motor vehicle is known from DE 100 45 776 A1. In this method the position of an object in relation to a motor vehicle is determined on the one hand by signal delay times and on the other hand by determining a direction from which the signals are received. A predetermined roaming area of an object is defined in this case as the authorization access to a motor vehicle.
A method for verifying an identification transmitter is known from DE 100 46 897 B4. With this method both the transmit field strength and also the transmit direction are detected by at least one vehicle-side signal transmitter, depending on its position relative to the signal generator. The evaluation of the measured signals described demands measurement accuracy which in practice can only be implemented with difficulty.
An anti-theft system is known from WO 98/50652 A1 in which a request signal is emitted from a vehicle-side transmitter unit via two separate antennas.
This means that the request signal is transmitted in at least two different transmission channels, depending on the transmission parameters. The transmission parameters are varied depending on the response signal.
A magnetic sensor for an anti-theft system is known from DE 197 12 911 A1. In this case the angle of incidence of electromagnetic radiation emitted by a transmitter in the near field is detected. The magnetic field sensor consists of two conductor loops essentially arranged concentrically to each other.
Furthermore access authorization systems for motor vehicles are also known from EP 1 403 653 A1 and GB 2 371 137 A.
The disadvantage of the prior art is that the anti-theft devices described are susceptible to manipulation by third parties. The method known from DE 100 46 897 B4 also demands levels of measurement accuracy which are too high.